Add is_real functionality for factorial, Fix issue #8658

Factorial function don't have the is_real functionality (check or real) to check this Assumption.
factorial(x) is not known to be real when  'x' is non-negative.

With this PR, user can check if the given factorial is real or not.
Fixes #8658 

That's by default.
For more info see line 99
https://github.com/sympy/sympy/blob/master/sympy/functions/combinatorial/tests/test_comb_factorials.py